1. Field of the Invention
The present invention relates to composite articles of a hard and a soft component having improved adhesion to each other and a method for improving the adhesion. In detail, the hard component is selected from cellulose esters and the soft component is selected from thermoplastic elastomers.
2. Background/Prior art
Cellulose esters are polymers made from renewable energy sources which become more and more popular because of their natural origin. Cellulose esters are transparent which makes them a candidate of choice for applications where transparency and clarity is needed. Furthermore, cellulose esters can be plastisized which increases their toughness at low temperatures. One representative cellulose ester is cellulose propionate.
There are many applications, for instance, in the automobile industry or for mechanical rubber goods, in which a combination of flexible and rigid materials is required. In most cases, a soft component is adhered onto the rigid cellulose propionate. Actually, this can be achieved by putting an adhesive system between the surface and the hard component of the article. These methods, however, are inherently expensive since they require extensive laboratory time. For details it is referred to A. Van Meesche and C. Radar, Adhesion of Elastomeric Alloy Thermoplastic Vulcanizates, in `Elastomerics`, September 1987, pages 21 to 24 and J. P. Vander Kooi and L. A. Goettler, Bonding Olefinic Thermoplastic Elastomers, in `Rubber World`, May 1985. EP-A-0 718 347 discloses a method to adhere thermoplastic elastomer blends to polyester substrates by treating the surface of the substrate with a blocked diisocyanate and optionally an epoxy resin.
One way to reduce the cost of the manufacture of the article is, for instance, to use sequential injection molding, a process during which the soft part is over-molded onto the rigid or hard material, and vice-versa. This technology gives outstanding adhesion between the soft and the hard polymers if they are miscible or at least compatible. If the two polymers are incompatible, they do not adhere together.
Cellulose esters and, as an example, cellulose propionate, are known to be incompatible with any other polymer (see Paul, Polymer Blends, Vol. 1 and 2, Academic Press, New York, 1977).
Therefore it was an object of the present invention to provide a method for adhering polymers, and in detail non-polar thermoplastic elastomers to cellulose esters providing an outstanding adhesion when over-molded, co-molded, co-blow molded or co-extruded.